It is important for diabetics to regularly check their blood glucose level for controlling the blood glucose level. However, it is troublesome to frequently visit a medical institution for measuring blood glucose level. In light of this, portable handheld blood glucose level measuring apparatuses are used, whereby diabetics can easily and conveniently measure the blood glucose level even when they are away from home, for example.
In using such a portable blood glucose level measuring apparatus, a glucose sensor for providing an enzyme reaction system is mounted to the blood glucose level measuring apparatus, and blood (analyte) is supplied to the glucose sensor for measuring the blood glucose level. Generally, in this case, the skin of the measurer is cut to extract blood, and the blood is supplied to the glucose sensor as the sample liquid. In this method, to lessen the burden on the measurer caused by the blood extraction, it is preferable that the amount of blood to be extracted is small. Therefore, various improvements have been made to enable the blood glucose level measurement by using a relatively small amount of blood (analyte).
For example, the glucose sensor comprises a substrate on which electrodes and a reagent layer are formed, and a capillary formed to accommodate the reagent layer (See FIGS. 2 and 3). The reagent layer includes oxidoreductase and an electron carrier. Generally, GOD or PQQGDH is used as the oxidoreductase, whereas potassium ferricyanide is used as the electron carrier (See JP-A 2000-65778, for example). In the glucose sensor, when the analyte is supplied to the reagent layer by using the capillary, a reaction system in a liquid phase is established in the capillary. Thus, by the oxidoreductase, oxidation reaction of e.g. glucose is catalyzed, while reduction reaction of the electron carrier is catalyzed.
In the portable blood glucose level measuring apparatus, a voltage is applied to the reaction system by using the electrodes of the glucose sensor, and the responsive current is measured. The responsive current depends on the amount of e.g. electron carrier in the reduced form (which relates to the glucose level), and utilized as the basis for computing the glucose level. The glucose level is computed by coulometry or amperometry. Coulometry is a technique in which most part of glucose in the analyte is subjected to reaction for obtaining the integrated value so that the glucose level is computed based on the integrated value (total electricity). Amperometry is a technique in which the responsive current is measured after a certain time period has elapsed from the start of the reaction so that the glucose level is computed based on the responsive current.
The reaction rate of GOD with glucose is low (Km (Michaelis constant) is high). Therefore, when coulometry is utilized in which most part of glucose in the analyte is subjected to reaction for obtaining the total electricity for computation, the measurement time becomes considerably long. Therefore, amperometry is utilized to measure the glucose level in a short period of time by using GOD as oxidoreductase.
In amperometry, however, when the glucose level is low, the enzyme reaction maybe almost completed before the responsive current is measured. In such a case, a low responsive current is measured, so that the measurement accuracy in a low concentration range is deteriorated. Further, the similar problem may occur when the amount of the analyte is considerably small, because the absolute amount of glucose is small. Such a problem may be solved by reducing the amount of enzyme to be used. However, when the amount of enzyme is small, the reaction rate of glucose is decreased. Therefore, for the analytes whose glucose levels are higher than a certain level, the difference in glucose level does not appear significantly as the difference in responsive current. As a result, when the amount of enzyme is reduced, the resolving power in the high concentration range is reduced, because the difference in glucose level cannot appear as the difference in responsive current. Therefore, amperometry is not suitable for the measurement for a small measurement range by the use of a small amount of analyte.
Moreover, the reactivity of GOD with the electron carrier is not so high. Therefore, to shorten the measurement time, a large amount of electron carrier need be used. As a result, the size reduction of the glucose sensor (reagent and capillary, to be exact) is difficult, so that the amount of analyte necessary for the measurement increases. Also from this point, the use of GOD is not suitable for the measurement of a small amount of analyte.
Under the above-described circumstances, it is said that accurate glucose level measurement by amperometry using GOD is possible only when the amount of analyte is no less than 0.6 μL which is converted to the measurement time of no less than 15 seconds and when the glucose level lies in the measurement range of 10 to 600 mg/dL.
It is known that, by coulometry using PQQGDH as oxidoreductase, the measurement of the blood glucose level is possible even with a minute amount, e.g. 0.3 μL of analyte. However, since coulometry is a technique in which most part of glucose in the analyte is used to compute the glucose level as noted above, the measurement time in a high glucose concentration range tends to become long as compared with amperometry. For example, to assure the practically necessary minimum measurement range (10-600 mg/dL), the measurement time of at least 15 to 30 seconds need be taken.
To shorten the measurement time, it may be one way to increase the content of enzyme and electron carrier in the reagent. In this case, however, the solubility of the reagent layer is reduced. Therefore, when the analyte is supplied to the capillary, it is difficult to form a reaction system in a uniform liquid phase in the capillary. As a result, due to the variation in the degree of dissolution among glucose sensors (or among measurements), the reproducibility is deteriorated or the influence of blood cell components in the blood increases, whereby the measurement accuracy is deteriorated. Particularly, since potassium ferricyanide has a low solubility to blood, the use of potassium ferricyanide as the electron carrier considerably deteriorates the measurement accuracy. Moreover, potassium ferricyanide has a low storage stability and is easily transferred to a reduced form. Therefore, an increase in the content of potassium ferricyanide leads to an increase in the background, whereby the measurement accuracy in a low glucose concentration range is deteriorated.